The trend in microelectronics, as well as in micromechanics, is towards ever smaller dimensions. Some of the most interesting techniques for fabrication of micro and submicro structures include different types of lithography.
Photolithography typically involves the steps of coating a substrate with a photoresist material to form a resist layer on a surface of the substrate. The resist layer is then exposed to radiation at selective portions, preferably by using a mask. Subsequent developing steps remove portions of the resist, thereby forming a pattern in the resist corresponding to the mask. The removal of resist portions exposes the substrate surface, which may be processed by e.g. etching, doping, or metallization. For fine scale replication, photolithography is limited by diffraction, which is dependent on the wavelength of the radiation used. For fabrication of structures on a scale of less than 50 nm, such a short wavelength is needed that the material requirements on the optical systems will be major.
An alternative technique is imprint technology. In an imprint lithography process, a substrate to be patterned is covered by a mouldable layer. A pattern to be transferred to the substrate is predefined in three dimensions on a stamp or template. The template is brought into contact with the mouldable layer, and the layer is softened, preferably by heating. The template is then pressed into the softened layer, thereby making an imprint of the template pattern in the mouldable layer. The layer is cooled down until it hardens to a satisfactory degree followed by detachment and removal of the template. Subsequent etching may be employed to replicate the template pattern in the substrate. The steps of heating and cooling the combined template and substrate can bring about movement in the engaging surfaces due to heat expansion. The larger the area to be imprinted, the larger the actual expansion and contraction, which can make the imprint process more difficult for larger surface areas.
A different form of imprint technology, generally known as step and flash imprint lithography has been proposed by Willson et al. in U.S. Pat. No. 6,334,960, and also by Mancini et al in U.S. Pat. No. 6,387,787. Similar to the imprint technique briefly described above, this technique involves a template having a structured surface defining a pattern to be transferred to a substrate. The substrate is covered by a layer of polymerisable fluid, a pre-polymer, into which layer the template is pressed such that the fluid fills recesses in the pattern structure. The template is made from a material which is transparent to a radiation wavelength range which is usable for polymerising the polymerisable fluid, typically UV light. By applying radiation to the fluid through the template, the fluid is solidified. The template is subsequently removed, after which the pattern thereof is replicated in the solid polymer material layer made from the polymerised fluid. Further processing transfers the structure in the solid polymer material layer to the substrate.
WO 02/067055 to Board of Regents, the University of Texas System, discloses a system for applying step and flash imprint lithography. Among other things, this document relates to production-scale implementation of a step and flash apparatus, also called a stepper. The template used in such an apparatus has a rigid body of transparent material, typically quartz. The template is supported in the stepper by flexure members, which allow the template to pivot about both X and Y axes, which are mutually perpendicular in a plane parallel to the substrate surface to be imprinted. This mechanism also involves a piezo actuator for controlling parallelism and the gap between the template and the substrate. Such a system is, however, not capable of handling large area substrate surfaces in a single imprint step. A step and flash system offered on the market is the IMPRIO 100, provided by Molecular Imprints, Inc, 1807-C West Braker Lane, Austin, Tex. 78758, U.S.A. This system has a template image area of 25×25 mm, and a street width of 0.1 mm. Although this system is capable of handling substrate wafers of up to 8 inches, the imprint process has to be repeated by lifting the template, moving it sideways, and lowering it to the substrate again, by means of an X-Y translation stage. Furthermore, for each such step, renewed alignment as well as new dispensation of polymerisable fluid has to be performed. This technique is therefore very time-consuming, and less than optimum for large scale production. Furthermore, besides problems of repeated alignment errors, and high accuracy demands on the translation stage, this technique suffers from the drawback that continuous structures which are larger than said template size cannot be produced. All in all, this means the productions costs may be too high to make this technique interesting for large scale production of fine structure devices.
Another drawback with the state of the art technology for UV-assisted imprint, is that in many cases it is desirable to use non-transparent templates. Nickel is typically used as a template material, for its excellent material properties. However, a nickel template is of course not transparent, wherefore UV radiation has to be supplied through the substrate. In such a case, a substrate of e.g. glass or quartz, or a suitable plastic material may be used. Furthermore, using different materials in the template and the substrate generally means that they have different coefficients of thermal expansion. This, in turn, may cause problems during steps of heating and cooling, limiting the accuracy of the process.